Add a MIME type for Tektronix rf5 files and register Wireshark for that type of
file.

The magic matching comes from wiretap/k12.c.

(Yes, I just made the application/x-tektronix-rf5 MIME type up.)

Change-Id: Iafd7300aa7f2664bb06e1330caede10785c21d68
Reviewed-on: https://code.wireshark.org/review/5607
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>